Internet Engineering Task Force (IETF) A. Makela
Request for Comments: 6521 Aalto University/Comnet
Category: Experimental J. Korhonen
ISSN: 2070-1721 Nokia Siemens Networks
February 2012
Home Agent-Assisted Route Optimization between Mobile IPv4 Networks
Abstract
This document describes a home agent-assisted route optimization
functionality for the IPv4 Network Mobility Protocol. The function
is designed to facilitate optimal routing in cases where all nodes
are connected to a single home agent; thus, the use case is route
optimization within a single organization or similar entity. The
functionality enables the discovery of eligible peer nodes (based on
information received from the home agent) and their network prefixes,
and the establishment of a direct tunnel between such nodes.
Status of This Memo
This document is not an Internet Standards Track specification; it is
published for examination, experimental implementation, and
evaluation.
This document defines an Experimental Protocol for the Internet
community. This document is a product of the Internet Engineering
Task Force (IETF). It represents the consensus of the IETF
community. It has received public review and has been approved for
publication by the Internet Engineering Steering Group (IESG). Not
all documents approved by the IESG are a candidate for any level of
Internet Standard; see Section 2 of RFC 5741.
Information about the current status of this document, any errata,
and how to provide feedback on it may be obtained at
http://www.rfc-editor.org/info/rfc6521.
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Copyright Notice
Copyright (c) 2012 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents
(http://trustee.ietf.org/license-info) in effect on the date of
publication of this document. Please review these documents
carefully, as they describe your rights and restrictions with respect
to this document. Code Components extracted from this document must
include Simplified BSD License text as described in Section 4.e of
the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License.
Table of Contents
1. Introduction and Motivations ....................................3
2. Terms and Definitions ...........................................6
3. Mobile IPv4 Route Optimization between Mobile Networks ..........8
3.1. Maintaining Route Optimization Information .................9
3.1.1. Advertising Route-Optimizable Prefixes ..............9
3.1.2. Route Optimization Cache ...........................11
3.2. Return Routability Procedure ..............................13
3.2.1. Router Keys ........................................15
3.2.2. Nonces .............................................15
3.2.3. Updating Router Keys and Nonces ....................16
3.3. Mobile-Correspondent Router Operations ....................16
3.3.1. Triggering Route Optimization ......................17
3.3.2. Mobile Router Routing Tables .......................17
3.3.3. Inter-Mobile Router Registration ...................18
3.3.4. Inter-Mobile Router Tunnels ........................20
3.3.5. Constructing Route-Optimized Packets ...............21
3.3.6. Handovers and Mobile Routers Leaving Network .......21
3.4. Convergence and Synchronization Issues ....................22
4. Data Compression Schemes .......................................23
4.1. Prefix Compression ........................................23
4.2. Realm Compression .........................................25
4.2.1. Encoding of Compressed Realms ......................25
4.2.2. Searching Algorithm ................................27
4.2.3. Encoding Example ...................................27
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5. New Mobile IPv4 Messages and Extensions ........................30
5.1. Mobile Router Route Optimization Capability Extension .....30
5.2. Route Optimization Reply ..................................31
5.3. Mobile-Correspondent Authentication Extension .............32
5.4. Care-of Address Extension .................................33
5.5. Route Optimization Prefix Advertisement Extension .........34
5.6. Home Test Init Message ....................................36
5.7. Care-of Test Init Message .................................36
5.8. Home Test Message .........................................37
5.9. Care-of Test Message ......................................38
6. Special Considerations .........................................39
6.1. NATs and Stateful Firewalls ...............................39
6.2. Handling of Concurrent Handovers ..........................40
6.3. Foreign Agents ............................................40
6.4. Multiple Home Agents ......................................40
6.5. Mutualness of Route Optimization ..........................41
6.6. Extensibility .............................................42
6.7. Load Balancing ............................................43
7. Scalability ....................................................43
8. Example Signaling Scenarios ....................................44
8.1. Registration Request ......................................44
8.2. Route Optimization with Return Routability ................45
8.3. Handovers .................................................46
9. Protocol Constants .............................................48
10. IANA Considerations ...........................................48
11. Security Considerations .......................................50
11.1. Return Routability .......................................50
11.2. Trust Relationships ......................................51
12. Acknowledgements ..............................................51
13. References ....................................................51
13.1. Normative References .....................................51
13.2. Informative References ...................................52
1. Introduction and Motivations
Traditionally, there has been no method for route optimization in
Mobile IPv4 [RFC5944] apart from an early attempt [MIP-RO]. Unlike
Mobile IPv6 [RFC6275], where route optimization has been included
from the start, with Mobile IPv4, route optimization hasn't been
addressed in a generalized scope.
Even though general route optimization may not be of interest in the
scope of IPv4, there are still specific applications for route
optimization in Mobile IPv4. This document proposes a method to
optimize routes between networks behind Mobile Routers (MRs), as
defined by Network Mobility (NEMO) [RFC5177]. Although NAT and the
pending shortage of IPv4 addresses make widespread deployment of end-
to-end route optimization infeasible, using route optimization from
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MR to MR is still a practical scenario. Note that the method
specified in this document is only for route optimization between
MRs; any network prefix not advertised by an MR would still be routed
via the home agent, although an MR could advertise very large address
spaces, e.g., by acting as an Internet gateway.
A particular use case concerns setting up redundant yet economical
enterprise networks. Recently, a trend has emerged where customers
prefer to maintain connectivity via multiple service providers.
Reasons include redundancy, reliability, and availability issues.
These kinds of multihoming scenarios have traditionally been solved
by using such technologies as multihoming BGP. However, a more
lightweight and economical solution is desirable.
From a service provider perspective, a common topology for an
enterprise customer network consists of one to several sites
(typically headquarters and various branch offices). These sites are
typically connected via various Layer 2 technologies (ATM or Frame
Relay Permanent Virtual Circuits (PVCs)), MPLS VPNs, or Layer 3
site-to-site VPNs. With a Service Level Agreement (SLA), a customer
can obtain very reliable and well-supported intranet connectivity.
However, compared to the cost of "consumer-grade" broadband Internet
access, the SLA-guaranteed version can be considered very expensive.
These consumer-grade options, however, are not a reliable approach
for mission-critical applications.
Mobile IP, especially MRs, can be used to improve reliability of
connectivity even when implemented over consumer-grade Internet
access. The customer becomes a client for a virtual service
provider, which does not take part in the actual access technology.
The service provider has a backend system and an IP address pool that
it distributes to customers. Access is provided by multiple,
independent, possibly consumer-grade ISPs, with Mobile IP providing
seamless handovers if service from a specific ISP fails. The
drawback of this solution is that it creates a star topology; all
Mobile IP tunnels end up at the service provider-hosted home agent,
causing a heavy load at the backend. Route optimization between
mobile networks addresses this issue, by taking the network load off
of the home agent and the backend.
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An example network is pictured below:
+----------------------------+
| Virtual Operator Backend |
+------------+ +-----+
| Home Agent | | AAA |
+------------+---------+-----+
|
.--.
_(. `)
_( ISP `)_
( Peering `)
( ` . Point ) )
`--(_______)--'
____ / | \
/ | \
.--. .--. .--.
_( `. _( `. _( `.
( ISP A ) ( ISP B ) ( ISP C )
( ` . ) ) ( ` . ) ) ( ` . ) )
`--(___.-' `--(___.-' `--(___.-'
| ______/ \ /
| / \ /
| / \ /
+----+ +----+
|MR A| |MR B|
+----+ +----+
| |
.--. .--.
_( `. _( `.
( Site A ) ( Site B )
( ` . ) ) ( ` . ) )
`--(___.-' `--(___.-'
Virtual Service Provider Architecture Using NEMOv4
In this example case, the organization network consists of two sites
that are connected via two ISPs for redundancy reasons. Mobile IP
allows fast handovers without the problems of multihoming and BGP
peering between each individual ISP and the organization. The
traffic, however, takes a non-optimal route through the virtual
operator backend.
Route optimization addresses this issue, allowing traffic between
Sites A and B to flow directly through ISP B's network, or in case of
a link failure, via the ISP peering point (such as the Metropolitan
Area Ethernet (MAE), e.g., MAE-West). The backend will not suffer
from heavy loads.
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The specification in this document is meant to be Experimental, with
the primary design goal of keeping the load on the backend to a
minimum. Additional design goals include extensibility to a more
generalized scope, such as not requiring all MRs to be homed on the
same home agent. Experiences are mostly sought regarding
applicability to real-world operations, and protocol-specific issues
such as signaling scalability, interworking with other Mobile IP
extensions not specifically addressed in this document, and behavior
of end-user applications over route-optimized paths.
The aforementioned use case is the original application. Moving this
specification to Standards Track should be considered after enough
deployment experience has been gathered. Besides the aforementioned
issues, additional elements that might require refinement based on
real-world experiences are delivery of information on networks
managed by peer MRs; conducting MR MR authentication; reaction
to, and recovery methods for, connectivity breakdowns and other
break-before-make topology changes; keepalive timer intervals;
formats of signaling extensions; behavior in NAT/firewalled
environments; and the prefix and realm compression algorithms.
2. Terms and Definitions
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in RFC 2119 [RFC2119].
Care-of Address (CoA)
RFC 5944 [RFC5944] defines a care-of address as the termination
point of a tunnel toward a mobile node, for datagrams forwarded to
the mobile node while it is away from home. The protocol can use
two different types of CoA: a "foreign agent care-of address",
which is an address of a foreign agent with which the mobile node
is registered, and a "co-located care-of address", which is an
externally obtained local address that the mobile node has
associated with one of its own network interfaces. However, in
the case of Network Mobility, foreign agents are not used, so no
foreign CoAs are used either.
Correspondent Router (CR)
RFC 5944 [RFC5944] defines a correspondent node as a peer with
which a mobile node is communicating. A CR is a peer MR that MAY
also represent one or more entire networks.
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Home Address (HoA)
RFC 5944 [RFC5944] defines a home address as an IP address that is
assigned for an extended period of time to a mobile node. It
remains unchanged regardless of where the node is attached to the
Internet.
Home Agent (HA)
RFC 5944 [RFC5944] defines a home agent as a router on a mobile
node's home network that tunnels datagrams for delivery to the
mobile node when it is away from home and maintains current
location information for the mobile node. For this application,
the "home network" sees limited usage.
Host Network Prefix
A host network prefix is a network prefix with a mask of /32,
e.g., 192.0.2.254/32, consisting of a single host.
Mobility Binding
RFC 5944 [RFC5944] defines Mobility Binding as the association of
an HoA with a CoA, along with the lifetime remaining for that
association.
Mobile Network Prefix
RFC 5177 [RFC5177] defines a mobile network prefix as the network
prefix of the subnet delegated to an MR as the mobile network.
Mobile Router (MR)
RFC 5177 [RFC5177] and RFC 5944 [RFC5944] define a mobile router
as a mobile node that can be a router that is responsible for the
mobility of one or more entire networks moving together, perhaps
on an airplane, a ship, a train, an automobile, a bicycle, or a
kayak.
Route Optimization Cache
A Route Optimization Cache is defined as a data structure,
maintained by MRs, containing possible destinations for route
optimization. The cache contains information (HoAs) on potential
CRs and their associated mobile networks.
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Return Routability (RR)
Return routability is defined as a procedure to bind an MR's HoA
to a CoA on a CR with a degree of trust.
| (Concatenation)
Some formulas in this specification use the symbol "|" to indicate
bytewise concatenation, as in A | B. This concatenation requires
that all of the octets of the datum A appear first in the result,
followed by all of the octets of the datum B.
First (size, input)
Some formulas in this specification use a functional form "First
(size, input)" to indicate truncation of the "input" data so that
only the first "size" bits remain to be used.
3. Mobile IPv4 Route Optimization between Mobile Networks
This section describes the changed functionality of the HA and the MR
compared to the base NEMOv4 operation defined in [RFC5177]. The
basic premise is still the same; MRs, when registering with the HA,
may inform the HA of the mobile network prefixes they are managing
(explicit mode), or the HA already knows the prefix assignments.
However, instead of prefix MR mapping information only remaining
on the HA and the single MR, this information will now be distributed
to the other MRs as well.
Home agent-assisted route optimization is primarily intended for
helping to optimize traffic patterns between multiple sites in a
single organization or administrative domain; however, extranets can
also be reached with optimized routes, as long as all MRs connect to
the same HA. The procedure aims to maintain backward compatibility;
with legacy nodes or routers, full connectivity is always preserved,
even though optimal routing cannot be guaranteed.
The scheme requires an MR to be able to receive messages from other
MRs unsolicited -- that is, without first initiating a request. This
behavior -- accepting unsolicited messages -- is similar to the
registration revocation procedure [RFC3543]. Many of the mechanisms
are the same, including the fact that advertising route optimization
support upon registration implies the capability to receive
Registration Requests and Return Routability messages from other MRs.
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Compared to IPv6, where mobile node correspondent node bindings
are maintained via Mobility Routing header and home address options,
Mobile IPv4 always requires the use of tunnels. Therefore,
inter-mobile-router tunnel establishment has to be conducted.
3.1. Maintaining Route Optimization Information
During registration, a registering MR MAY request information on
route-optimizable network prefixes. The MR MAY also allow
redistribution of information on its managed network prefixes
regardless of whether they are explicitly registered or already
configured. These are indicated with a Mobile Router Route
Optimization Capability Extension; see Section 5.1. If the HA
accepts the request for route optimization, this is indicated with a
Route Optimization Reply Extension (Section 5.2) in the Registration
Reply.
Note that the redistribution of network prefix information from the
HA happens only during the registration signaling. There are no
"routing updates" from the HA except during re-registrations
triggered by handovers, registration timeouts, and specific
solicitation. The solicitation re-registration MAY occur if a CR
receives a Registration Request from an unknown MR (see
Section 3.3.3).
3.1.1. Advertising Route-Optimizable Prefixes
As noted, an HA that supports NEMO already maintains information on
which network prefixes are reachable behind specific MRs. The only
change to this functionality is that this information can now be
distributed to other MRs upon request. This request is implied by
including a Route Optimization Capability Extension (Section 5.1) and
setting the 'R' bit.
When an HA receives a Registration Request, standard authentication
and authorization procedures are conducted.
If registration is successful and the Route Optimization Capability
Extension was present in the Registration Request, the reply message
MUST include the Route Optimization Reply Extension (Section 5.2) to
indicate that the Route Optimization Capability Extension was
understood. Furthermore, the extension also informs the MR whether
NAT was detected between the HA and the MR using the procedure in
RFC 3519 [RFC3519], which is based on the discrepancy between the
requester's indicated CoA and the packet's source address.
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The reply message MAY also include one Route Optimization Prefix
Advertisement Extension, which informs the MR of existing mobile
network prefixes and the MRs that manage them, if eligible for
redistribution. The networks SHOULD be included in order of
priority, with the prefixes determined, by policy, as most desirable
targets for route optimization listed first. The extension is
constructed as shown in Section 5.5. The extension consists of a
list where each MR, identified by its HoA, is listed with
corresponding prefix(es) and their respective realm(s).
Each network prefix can be associated with a realm [RFC4282], usually
in the form 'organization.example.com'. Besides the routers in the
customer's own organization, the prefix list may also include other
MRs, e.g., a default prefix (0.0.0.0/0) pointing toward an Internet
gateway for Internet connectivity or additional prefixes belonging to
possible extranets. The realm information can be used to make policy
decisions on the MR, such as preferring optimization within a
specific realm only. Furthermore, the unique realm information can
be used to differentiate between overlapping address spaces utilized
by the same or different organizations concurrently and adjusting
forwarding policies accordingly.
In a typical scenario, where network prefixes are allocated to MRs
connecting to a single HA, the prefixes are usually either continuous
or at least very close to each other. Due to these characteristics,
an optional prefix compression mechanism is provided. Another
optional compression scheme is in use for realm information, where
realms often share the same higher-level domains. These compression
mechanisms are further explained in Section 4.
Upon receiving a Registration Reply with a Route Optimization Prefix
Advertisement Extension, the MR SHALL insert the MR HoAs included in
the extension as host-prefixes to the local Route Optimization Cache
if they do not already exist. If present, any additional prefix
information SHALL also be inserted into the Route Optimization Cache.
The MR MAY discard entries from a desired starting point onward, due
to memory or other policy-related constraints. The intention of
listing the prefixes in order of priority is to provide implicit
guidance for this decision. If the capacity of the device allows,
the MR SHOULD use information on all advertised prefixes.
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3.1.2. Route Optimization Cache
MRs supporting route optimization will maintain a Route Optimization
Cache.
The Route Optimization Cache contains mappings between potential CR
HoAs, network(s) associated with each HoA, information on
reachability related to NAT and other divisions, and information
related to the RR procedure. The cache is populated based on
information received from the HA in Route Optimization Prefix
Advertisement Extensions and in registration messages from CRs.
Portions of the cache may also be configured statically.
The Route Optimization Cache contains the following information for
all known CRs. Note that some fields may contain multiple entries.
For example, during handovers, there may be both old and new CoAs
listed.
CR-HoA
Correspondent router's home address. Primary key identifying
each CR.
CR-CoA(s)
Correspondent router's care-of address(es). May be empty if none
known. Potential tunnel's destination address(es).
MR-CoA
Mobile router's care-of address currently used with this CR.
Tunnel's source address.
Tunnels
Tunnel interface(s) associated with this CR. The tunnel interface
itself handles all the necessary operations to keep the tunnel
operational, e.g., sending keepalive messages required by UDP
encapsulation.
NAT states
A table of booleans. Contains entries for all pairs of potential
MR-CoAs and CR-CoAs that are known to require NAT awareness. The
table is populated either statically or based on information
received during operation. A setting of true indicates that the
MR can establish a UDP tunnel toward the CR, using this pair of
CoAs. A received advertisement can indicate that the value should
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be set to false for all of the respective CR's CoAs. Settings in
this table affect tunnel establishment direction; see
Section 3.3.4 and the registration procedure when deciding which
CoAs to include in the Care-of Address Extension in the
Registration Reply. The existence of an entry mandates the use of
UDP encapsulation.
RRSTATEs
Return routability state for each CR-HoA - MR-CoA pair. States
are INACTIVE, IN PROGRESS, and ACTIVE. If state is INACTIVE, the
RR procedure must be completed before forwarding route-optimized
traffic. If state is IN PROGRESS or ACTIVE, the information
concerning this CR MUST NOT be removed from the Route Optimization
Cache as long as a tunnel to the CR is established.
KRms
Registration management key for each CR-HoA - MR-CoA pair. This
field is only used if configured statically -- if the KRm was
computed using the RR procedure, it is calculated in situ based on
nonces and the router key. If configured statically, RRSTATE is
permanently set to ACTIVE.
Care-of nonce indices
If the KRm was established with the RR procedure, contains the
care-of nonce index for each MR-CoA - CR-HoA pair.
Care-of keygen token
If the KRm was established with the RR procedure, contains the
care-of keygen token for each MR-CoA - CR-HoA pair.
Home nonce indices
If the KRm was established with the RR procedure, contains the
Home nonce index for each CR-HoA.
Home keygen token
If the KRm was established with the RR procedure, contains the
home keygen token for each CR-HoA.
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Network prefixes
A list of destination network prefixes reachable via this CR.
Includes network and prefix length, e.g., 192.0.2.0/25. Always
contains at least a single entry: the CR-HoA host network prefix
in the form of 192.0.2.1/32.
Realms
Each prefix may be associated with a realm. May also be empty, if
the realm is not provided by advertisement or configuration.
Prefix_Valid
Boolean field for each prefix - CR-HoA pair, which is set to true
if this prefix's owner has been confirmed. The host network
prefix consisting of the CR itself does not need validation beyond
the RR procedure. For other prefixes, the confirmation is done by
soliciting the information from the HA. Traffic for prefixes that
have unconfirmed ownership should not be routed through the
tunnel.
Information that is no longer valid due to expirations or topology
changes MAY be removed from the Route Optimization Cache as desired
by the MR.
3.2. Return Routability Procedure
The purpose of the RR procedure is to establish CoA HoA bindings
in a trusted manner. The RR procedure for Mobile IPv6 is described
in [RFC6275]. The same principles apply to the Mobile IPv4 version:
two messages are sent to the CR's HoA -- one via the HA using the
MR's HoA, and the other directly from the MR's CoA, with two
responses coming through the same routes. The registration
management key is derived from token information carried on these
messages. This registration management key (KRm) can then be used to
authenticate Registration Requests (comparable to Binding Updates in
Mobile IPv6).
The RR procedure is a method provided by Mobile IP to establish the
KRm in a relatively lightweight fashion. If desired, the KRms can be
configured on MRs statically, or by using a desired external secure
key provisioning mechanism. If KRms are known to the MRs via some
other mechanism, the RR procedure can be skipped. Such provisioning
mechanisms are out of scope for this document.
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The main assumption on traffic patterns is that the MR that initiates
the RR procedure can always send outbound messages, even when behind
a NAT or firewall. This basic assumption made for NAT Traversal in
[RFC3519] is also applicable here. In the case where the CR is
behind such obstacles, it receives these messages via the reverse
tunnel to the CR's HoA; thus, any problem regarding the CR's
connectivity is addressed during registration with the HA.
The RR procedure consists of four Mobile IP messages: Home Test Init
(HoTI), Care-of Test Init (CoTI), Home Test (HoT), and Care-of Test
(CoT). They are constructed as shown in Sections 5.6 through 5.9.
If the MR has included the Mobile Router Route Optimization
Capability Extension in its Registration Request, it MUST be able to
accept Return Routability messages. The messages are delivered as
Mobile IP signaling packets. The destination address of the HoTI and
CoTI messages is set to the CR's HoA, with the sources being the MR's
HoA and CoA, respectively.
The RR procedure begins with the MR sending HoTI and CoTI messages,
each containing a (different) 64-bit random value -- the cookie. The
cookie is used to bind a specific signaling exchange together.
Upon receiving the HoTI or CoTI message, the CR MUST have a secret
correspondent router key (Kcr) and nonce. If it does not have this
material yet, it MUST produce it before continuing with the RR
procedure.
The CR responds to HoTI and CoTI messages by constructing HoT and CoT
messages, respectively, as replies. The HoT message contains a home
init cookie, current home nonce index, and home keygen token. The
CoT message contains a care-of init cookie, current care-of nonce
index, and care-of keygen token.
The home keygen token is constructed as follows:
Home keygen token = First (64, HMAC_SHA1 (Kcr, (home address |
nonce | 0)))
The care-of keygen token is constructed as follows:
Care-of keygen token = First (64, HMAC_SHA1 (Kcr, (care-of address |
nonce | 1)))
Note that the CoA in this case is the source address of the received
CoTI message packet. The address may have changed in transit due to
network address translation. This does not affect the registration
process; subsequent Registration Requests are expected to arrive from
the same translated address.
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The RR procedure SHOULD be initiated when the Route Optimization
Cache's RRSTATE field for the desired CoA with the target CR is
INACTIVE. If the state was INACTIVE, the state MUST be set to IN
PROGRESS when the RR procedure is initiated. In the case of a
handover occurring, the MR SHOULD only send a CoTI message to obtain
a new care-of keygen token; the home keygen token may still be valid.
If the reply to a registration indicates that one or both of the
tokens have expired, the RRSTATE MUST be set to INACTIVE. The RR
procedure may then be restarted as needed.
Upon completion of the RR procedure, the Route Optimization Cache's
RRSTATE field is set to ACTIVE, allowing for Registration Requests to
be sent. The MR will establish a KRm. By default, this will be done
using the SHA1 hash algorithm, as follows:
KRm = SHA1 (home keygen token | care-of keygen token)
When de-registering (by setting the Registration Request's lifetime
to zero), the care-of keygen token is not used. Instead, the KRm is
generated as follows:
KRm = SHA1 (home keygen token)
As in Mobile IPv6, the CR does not maintain any state for the MR
until after receiving a Registration Request.
3.2.1. Router Keys
Each MR maintains a Kcr, which MUST NOT be shared with any other
entity. The Kcr is used for authenticating peer MRs in the situation
where an MR is acting as a CR. This is analogous to the node key
(Kcn) in Mobile IPv6. A CR uses its router key to verify that the
keygen tokens sent by a peer MR in a Registration Request are the
CR's own. The router key MUST be a random number, 16 octets in
length, generated with a good random number generator [RFC4086].
The MR MAY generate a new key at any time to avoid persistent key
storage. If desired, it is RECOMMENDED that the keys be expired in
conjunction with nonces; see Section 3.2.3.
3.2.2. Nonces
Each MR also maintains one or more indexed nonces. Nonces SHOULD be
generated periodically with a good random number generator [RFC4086].
The MR may use the same nonces with all MRs. Nonces MAY be of any
length, with the RECOMMENDED length being 64 bits.
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3.2.3. Updating Router Keys and Nonces
The router keys and nonce updating guidelines are similar to those
for Mobile IPv6. MRs keep both the current nonce and the small set
of valid previous nonces whose lifetimes have not expired yet. A
nonce should remain valid for at least MAX_TOKEN_LIFETIME seconds
(see Section 9) after it has first been used in constructing an RR
response. However, the CR MUST NOT accept nonces beyond
MAX_NONCE_LIFETIME seconds (see Section 9) after the first use. As
the difference between these two constants is 30 seconds, a
convenient way to enforce the above lifetimes is to generate a new
nonce every 30 seconds. The node can then continue to accept keygen
tokens that have been based on the last 8 (MAX_NONCE_LIFETIME / 30)
nonces. This results in keygen tokens being acceptable
MAX_TOKEN_LIFETIME to MAX_NONCE_LIFETIME seconds after they have been
sent to the mobile node, depending on whether the token was sent at
the beginning or end of the first 30-second period. Note that the
correspondent node may also attempt to generate new nonces on demand,
or only if the old nonces have been used. This is possible as long
as the correspondent node keeps track of how long ago the nonces were
used for the first time and does not generate new nonces on every
return routability request.
If the Kcr is being updated, the update SHOULD be done at the same
time as the nonce is updated. This way, nonce indexes can be used to
refer to both Kcrs and nonces.
3.3. Mobile-Correspondent Router Operations
This section deals with the operation of mobile and correspondent
routers performing route optimization. Note that in the context of
this document, all routers work as both MR and CR. The term "mobile
router" applies to the router initiating the route optimization
procedure, and "correspondent router" indicates the peer router.
There are two issues regarding IPv4 that are different when compared
to Mobile IPv6 route optimization. First of all, since Mobile IPv4
always uses tunnels, there must be a tunnel established between the
MR's and the CR's CoAs. The CR learns of the MR's CoA, because it is
included in the Registration Request. The MR learns the CR's CoA via
a new extension, "Care-of Address", in the Registration Reply. The
second issue is a security consideration: In a Registration Request,
the MR claims to represent an arbitrary IPv4 network. If the CR has
not yet received this information (HoA network prefix), it SHOULD
perform a re-registration with the HA to verify the claim.
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An additional aspect is that the MR MAY use a different CoA for
different CRs (and the HA). This is useful in situations where the
network provides only partial-mesh connectivity and specific
interfaces must be used to reach specific destinations. In addition,
this allows for load balancing.
3.3.1. Triggering Route Optimization
Since each MR knows the eligible route-optimizable networks, the
route optimization between all CRs can be established at any time;
however, a better general practice is to conduct route optimization
only on demand. It is RECOMMENDED that route optimization be started
only when sending a packet that originates from a local managed
network (and if the network is registered as route optimizable) and
whose destination address falls within the network prefixes of the
Route Optimization Cache. With a small number of MRs, such on-demand
behavior may not be necessary, and full-mesh route optimization may
be in place constantly.
3.3.2. Mobile Router Routing Tables
Each MR maintains a routing table. In a typical situation, the MR
has one or more interface(s) to the local networks, one or more
interface(s) to wide-area networks (such as those provided by ISPs),
and a tunnel interface to the HA. Additional tunnel interfaces
become activated as route optimization is being performed.
The routing table SHOULD typically contain network prefixes managed
by CRs associated with established route-optimized tunnel interfaces.
A default route MAY point to the reverse tunnel to the HA if not
overridden by prefix information. The routing table MAY also include
additional routes if required by the tunneling implementation.
The routes for the HoAs of any CRs SHOULD also be pointing toward
their respective tunnels that are using the optimized path.
If two prefixes overlap each other, e.g., 192.0.2.128/25 and
192.0.2.128/29, the standard longest-match rule for routing is in
effect. However, overlapping private addresses SHOULD be considered
an error situation. Any aggregation for routes in private address
space SHOULD be conducted only at the HA.
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3.3.3. Inter-Mobile Router Registration
If route optimization between an MR and a CR is desired, either the
RR procedure must have been performed (see Section 3.2), or the KRm
must be pre-shared between the MR and the CR. If either condition
applies, an MR MAY send a Registration Request to the CR's HoA from
the desired interface.
The Registration Request's Source Address and Care-of Address fields
are set to the address of the desired outgoing interface on the MR.
The address MAY be the same as the CoA used with the HA. The Home
Agent field is set to the HA of the MR. The Registration Request
MUST be sent to (have a destination address of) the HoA of the CR.
The Registration Request MUST include a Mobile-Correspondent
Authentication Extension (defined in Section 5.3) and SHOULD include
a Mobile Network Request Extension (defined in [RFC5177]). If
present, the Mobile Network Request Extension MUST contain the
network prefixes, as if registering in explicit mode. If timestamps
are used, the CR MUST check the Identification field for validity.
The Authenticator field is hashed with the KRm.
The CR replies to the request with a Registration Reply. The
Registration Reply MUST include a Mobile-Correspondent Authentication
Extension (defined in Section 5.3) and, if a Mobile Network Request
Extension was present in the request, a Mobile Network
Acknowledgement Extension.
The encapsulation can be set as desired, except in the case where the
Route Optimization Cache Entry has NAT entries for the CR, or the MR
itself is known to be behind a NAT or firewall. If either condition
applies, the Registration Request MUST specify UDP encapsulation. It
is RECOMMENDED that UDP encapsulation always be used to facilitate
detection of path failures via a keepalive mechanism.
The CR first checks the Registration Request's authentication against
Kcr and nonce indexes negotiated during the RR procedure. This
ensures that the Registration Request is coming from a valid MR. If
the check fails, an appropriate Registration Reply code is sent (see
Section 10). If the failure is due to the nonce index expiring, the
MR sets RRSTATE for the CR to INACTIVE. The RR procedure MAY then be
initiated again.
If the check passes, the CR MUST then check its Route Optimization
Cache to determine whether the MR exists and is associated with the
prefixes included in the request (i.e., whether prefixes are present
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and the 'HA' flag is true for each prefix). Note that the viewpoint
is always local; the CR compares CR-HoA entries against the MR's HoA
-- from the CR's perspective, the MR is also a "correspondent
router".
If the check against the cache fails, the CR SHOULD send a
re-Registration Request to the HA with the 'S' (solicitation) bit
set, thus obtaining the latest information on network prefixes
managed by the incoming MR. If, even after this update, the prefixes
still don't match, the reply's Mobile Network Acknowledgement code
MUST be set to "MOBNET_UNAUTHORIZED". The registration MAY also be
rejected completely. This verification is done to protect against
MRs claiming to represent arbitrary networks; however, since the HA
is assumed to provide trusted information, it can authorize the MR's
claim. If the environment itself is considered trusted, the CR can,
as a policy, accept registrations without this check; however, this
is NOT RECOMMENDED as a general practice.
If the prefixes match, the CR MAY accept the registration. If the CR
chooses to accept, the CR MUST check to determine if a tunnel to the
MR already exists. If the tunnel does NOT exist or has wrong
endpoints (CoAs), a new tunnel MUST be established and the Route
Optimization Cache updated. The reply MUST include a list of
eligible CoAs (see Section 5.4) with which the MR may establish a
tunnel. The reply MUST also include the Mobile-Correspondent
Authentication Extension (see Section 5.3).
Upon receiving the Registration Reply, the MR MUST check to determine
if a tunnel to the CR already exists. If the tunnel does NOT exist
or has wrong endpoints (CoAs), a new tunnel MUST be established and
the Route Optimization Cache updated. This is covered in detail in
Section 3.3.4.
The CR's routing table MUST be updated to indicate that the MR's
networks are reachable via the direct tunnel to the MR.
After the tunnel is established, the MR MAY update its routing tables
to reach all of the CR's Prefixes via the tunnel, although it is
RECOMMENDED that time be given for the CR to perform its own,
explicit registration. This is primarily a policy decision,
depending on the network environment. See Section 6.5.
Due to the fact that the route optimization procedures may occur
concurrently at both MRs, each working as each other's CR, there may
be a situation where two routers are attempting to establish separate
tunnels between them at the same time. If a router with a smaller
HoA (meaning a normal 32-bit integer comparison treating IPv4
addresses as 32-bit unsigned integers) receives a Registration
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Request (in the CR role) while its own Registration Request (sent in
the MR role) is pending, the attempt should be accepted with reply
code "concurrent registration" (Value 2). If receiving such an
indication, the recipient SHOULD consider the registration a success
but only act on it once the peer has completed its own registration.
3.3.4. Inter-Mobile Router Tunnels
Inter-MR tunnel establishment follows establishing standard reverse
tunnels to the HA. The Registration Request to the CR includes
information on the desired encapsulation. It is RECOMMENDED that UDP
encapsulation be used. In the cases of Generic Router Encapsulation
(GRE) [RFC2784], IP over IP [RFC2003], or minimal encapsulation
[RFC2004], no special considerations regarding reachability are
necessary. The tunnel has no stateful information; the packets are
simply encapsulated within the GRE, IP, or minimal header.
The tunnel origination point for the CR is its CoA, not the HoA where
the Registration Requests were sent. This is different from the
creation of the reverse tunnel to the HA, which reuses the channel
from registration signaling.
Special considerations rise from using UDP encapsulation, especially
in cases where one of the MRs is located behind a NAT or firewall. A
deviation from RFC 3519 [RFC3519] is that keepalives should be sent
from both ends of the tunnel to detect path failures after the
initial keepalive has been sent -- this allows both the MR and CR to
detect path failures.
The initial UDP keepalive SHOULD be sent by the MR. Only after the
first keepalive is successfully completed SHOULD the tunnel be
considered eligible for traffic. If a reply to the initial keepalive
is not received, the MR may opt to attempt sending the keepalive to
other CoAs provided by the Registration Reply to check whether they
provide better connectivity; or, if all of these fail, the MR may
perform a re-registration via an alternative interface, or deregister
completely. See Section 6.1. Once the initial keepalive packet has
reached the CR and a reply has been sent, the CR MAY start sending
its own keepalives.
The original specification for UDP encapsulation suggests a keepalive
interval default of 110 seconds. However, to provide fast response
time and switching to alternate paths, it is RECOMMENDED, if power
and other constraints allow, that considerably shorter periods be
used, adapting to the perceived latency as needed. However, the
maximum amount of keepalives SHOULD at no point exceed
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MAX_UPDATE_RATE times per second. The purpose of the keepalive is
not to keep NAT or firewall mappings in place but to serve as a
mechanism to provide fast response in case of path failures.
If both the MR and the CR are behind separate NATs, route
optimization cannot be performed between them. Possible ways to set
up mutual tunneling when both routers are behind NATs are outside the
scope of this document. However, some of these issues are addressed
in Section 6.1.
The designations "MR" and "CR" only apply to the initial tunnel
establishment phase. Once a tunnel is established between two
routers, either of them can opt to either tear down the tunnel or
perform a handover. Signaling messages have to be authenticated with
a valid KRm.
3.3.5. Constructing Route-Optimized Packets
All packets received by the MR are forwarded using normal routing
rules according to the routing table. There are no special
considerations when constructing the packets; the tunnel interface's
own processes will encapsulate any packet automatically.
3.3.6. Handovers and Mobile Routers Leaving Network
Handovers and connection breakdowns can be categorized as either
ungraceful or graceful, also known as "break-before-make" (bbm) and
"make-before-break" (mbb) situations.
As with establishment, the "mobile router" discussed here is the
router wishing to change connectivity state, with the "correspondent
router" being the peer.
When an MR wishes to join its home link, it SHOULD, in addition to
sending the Registration Request to the HA with lifetime set to zero,
also send such a request to all known CRs, to their HoAs. The CR(s),
upon accepting this request and sending the reply, will check whether
the Route Optimization Cache contains any prefixes associated with
the requesting MR. These entries should be removed and the routing
table updated accordingly (traffic for the prefixes will be forwarded
via the HA again). The tunnel MUST then be destroyed. A short grace
period SHOULD be used to allow possible in-transit packets to be
received correctly.
In the case of a handover, the CR simply needs to update the tunnel's
destination to the MR's new CoA. The MR SHOULD keep accepting
packets from both old and new CoAs for a short grace period,
typically on the order of ten seconds. In the case of UDP
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encapsulation, it is RECOMMENDED that the same port numbers be used
for both registration signaling and tunneled traffic, if possible.
The initial keepalive message sent by the MR will verify that direct
connectivity exists between the MR and CR -- if the keepalive fails,
the MR SHOULD attempt alternate paths.
If the MR was unable to send the re-Registration Request before
handover, it MUST send it immediately after handover has been
completed and a tunnel with the HA is established. Since the
changing of CoA(s) invalidates the KRm, it is RECOMMENDED that
partial return routability be conducted by sending a CoTI message via
the new CoA and obtaining a new care-of keygen token. In all cases,
necessary tokens also have to be acquired if the existing tokens have
expired.
If a reply is not received for a Registration Request to a CR, any
routes to the network prefixes managed by the CR MUST be removed from
the routing table, thus causing the user traffic to be forwarded via
the HA.
3.4. Convergence and Synchronization Issues
The information the HA maintains on mobile network prefixes and the
MRs' Route Optimization Caches does not need to be explicitly
synchronized. This is based on the assumption that at least some of
the traffic between nodes inside mobile networks is always
bidirectional. If using on-demand route optimization, this also
implies that when a node in a mobile network talks to a node in
another mobile network, if the initial packet does not trigger route
optimization, the reply packet will.
Consider a situation with three mobile networks, A, B, and C, handled
by three mobile routers, MR A, MR B, and MR C, respectively. If they
register with an HA in this order, the situation goes as follows:
MR A registers and receives no information on other networks from the
HA, as no other MR has registered yet.
MR B registers and receives information on mobile network A being
reachable via MR A.
MR C registers and receives information on both of the other mobile
networks.
If a node in mobile network C is about to send traffic to mobile
network A, the route optimization is straightforward; MR C already
has network A in its Route Optimization Cache. Thus, packet
transmission triggers route optimization toward MR A. When MR C
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registers with MR A (after the RR procedure is completed), MR A does
not have information on mobile network C; thus, it will perform a
re-registration with the HA on demand. This allows MR A to verify
that MR C is indeed managing network C.
If a node in mobile network B sends traffic to mobile network C, MR B
has no information on network C. No route optimization is triggered.
However, when the node in network C replies and the reply reaches MR
C, route optimization happens as above. Further examples of
signaling are in Section 8.
Even in the very rare case of completely unidirectional traffic from
an entire network, re-registrations with the HA caused by timeouts
will eventually cause convergence. However, this should be treated
as a special case.
Note that all MRs are connected to the same HA. For possibilities
concerning multiple HAs, see Section 6.4.
4. Data Compression Schemes
This section defines the two compression formats used in Route
Optimization Prefix Advertisement Extensions.
4.1. Prefix Compression
Prefix compression is based on the idea that prefixes usually share
common properties. The scheme is simple delta compression. In the
prefix information advertisement (Section 5.5), the 'D' bit indicates
whether receiving a "master" or a "delta" prefix. This, combined
with the Prefix Length information, allows for compression and
decompression of prefix information.
If D = 0, what follows in the "Prefix" field are bits 1..n of the new
master prefix, where n is PLen. This is rounded up to the nearest
full octet. Thus, prefix lengths of /4 and /8 take 1 octet, /12 and
/16 take 2 octets, /20 and /24 take 3 octets, and longer prefix
lengths take a full 4 octets.
If D = 1, what follows in the "Prefix" field are bits m..PLen of the
prefix, where m is the first changed bit of the previous master
prefix, with padding from the master prefix filling the field to a
full octet. The maximum value of PLen - m is 8 (that is, the delta
MUST fit into one octet). If this is not possible, a new master
prefix has to be declared. If the prefixes are equal -- for example,
in the case where the same prefix appears in multiple realms -- then
one octet is still encoded, consisting completely of padding from the
master prefix.
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Determining the order of prefix transmission should be based on
saving maximum space during transmission.
An example of compression and transmitted data, where network
prefixes 192.0.2.0/28, 192.0.2.64/26, and 192.0.2.128/25 are
transmitted, is illustrated in Figure 1. Because of the padding to
full octets, redundant information is also sent. The bit patterns
being transmitted are as follows:
=+= shows the prefix mask
--- shows the master prefix for delta coded prefixes
192.0.2.0/28, D = 0
0 1 2 3
1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|1|1|0|0|0|0|0|0|.|0|0|0|0|0|0|0|0|.|0|0|0|0|0|0|1|0|.|0|0|0|0|0|0|0|0|
+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+-+-+-+-+
^ ^
+---------------------------- encoded ------------------------------+
^ ^
+-pad-+
192.0.2.64/26, D = 1
0 1 2 3
1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2
+-------------------------------------------------------------+-+-+-+-+
|1|1|0|0|0|0|0|0|.|0|0|0|0|0|0|0|0|.|0|0|0|0|0|0|1|0|.|0|1|0|0|0|0|0|0|
+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+-+-+-+-+-+-+
^ ^
+--- encoded ---+
^ ^
+-- padding --+
192.0.2.128/25, D = 1
0 1 2 3
1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2
+-------------------------------------------------------------+-+-+-+-+
|1|1|0|0|0|0|0|0|.|0|0|0|0|0|0|0|0|.|0|0|0|0|0|0|1|0|.|1|0|0|0|0|0|0|0|
+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+-+-+-+-+-+-+-+
^ ^
+--- encoded ---+
^ ^
+- padding -+
Figure 1: Prefix Compression Example
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The first prefix, 192.0.2.0/28, is considered a master prefix and is
transmitted in full. The PLen of 28 bits determines that all four
octets must be transmitted. If the prefix would have been, e.g.,
192.0.2.0/24, three octets would have sufficed, since 24 bits fit
into 3 octets.
For the following prefixes, D = 1. Thus, they are deltas of the
previous prefix, where D was zero.
192.0.2.64/26 includes bits 19-26 (full octet). Bits 19-25 are
copied from the master prefix, but bit 26 is changed to 1. The final
notation in binary is "1001", or 0x09.
192.0.2.128/25 includes bits 18-25 (full octet). Bits 18-24 are
copied from the master prefix, but bit 25 is changed to 1. The final
notation in binary is "101", or 0x05.
The final encoding thus becomes
+----------------+--------+-+---------------------+
| Prefix | PLen |D| Transmitted Prefix |
+----------------+--------+-+---------------------+
| 192.0.2.0/28 | 28 |0| 0xc0 0x00 0x02 0x00 |
| 192.0.2.64/26 | 26 |1| 0x09 |
| 192.0.2.128/25 | 25 |1| 0x05 |
+----------------+--------+-+---------------------+
It should be noted that in this case the order of prefix transmission
would not affect compression efficiency. If prefix 192.0.2.128/25
would have been considered the master prefix and the others as deltas
instead, the resulting encoding still fits into one octet for the
subsequent prefixes. There would be no need to declare a new master
prefix.
4.2. Realm Compression
4.2.1. Encoding of Compressed Realms
In order to reduce the size of messages, the system introduces a
realm compression scheme, which reduces the size of realms in a
message. The compression scheme is a simple dynamically updated
dictionary-based algorithm, which is designed to compress text
strings of arbitrary length. In this scheme, an entire realm, a
single label, or a list of labels may be replaced with an index to a
previous occurrence of the same string stored in the dictionary. The
realm compression defined in this specification was inspired by the
RFC 1035 [RFC1035] DNS domain name label compression scheme. Our
algorithm is, however, improved to gain more compression.
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When compressing realms, the dictionary is first reset and does not
contain a single string. The realms are processed one by one, so the
algorithm does not expect to see them all or the whole message at
once. The state of the compressor is the current content of the
dictionary. The realms are compressed label by label or as a list of
labels. The dictionary can hold a maximum of 128 strings; after
that, a rollover MUST occur, and existing contents will be
overwritten. Thus, when adding the 129th string into the dictionary,
the first entry of the dictionary MUST be overwritten, and the index
of the new string will become 0.
The encoding of an index to the dictionary or an uncompressed run of
octets representing a single label has purposely been made simple,
and the whole encoding works on an octet granularity. The encoding
of an uncompressed label takes the form of one octet as follows:
0
0 1 2 3 4 5 6 7
+-+-+-+-+-+-+-+-+-+-+-+-=================-+-+-+-+
|0| LENGTH | 'length' octets long string.. |
+-+-+-+-+-+-+-+-+-+-+-+-=================-+-+-+-+
This encoding allows label lengths from 1 to 127 octets. A label
length of zero (0) is not allowed. The "label length" tag octet is
then followed by up to 127 octets of the actual encoded label string.
The index to the dictionary (the "label index" tag octet) takes the
form of one octet as follows:
0
0 1 2 3 4 5 6 7
+-+-+-+-+-+-+-+-+
|1| INDEX |
+-+-+-+-+-+-+-+-+
The above encodings do not allow generating an output octet value of
zero (0). The encapsulating Mobile IPv4 extension makes use of this
property and uses the value of zero (0) to mark the end of the
compressed realm or to indicate an empty realm. It is also possible
to encode the complete realm using only "label length" tags. In this
case, no compression takes place. This allows the sender to skip
compression -- for example, to reduce computation requirements when
generating messages. However, the receiver MUST always be prepared
to receive compressed realms.
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4.2.2. Searching Algorithm
When compressing the input realm, the dictionary is searched for a
matching string. If no match could be found, the last label is
removed from the right-hand side of the used input realm. The search
is repeated until the whole input realm has been processed. If no
match was found at all, then the first label of the original input
realm is encoded using the "label length" tag, and the label is
inserted into the dictionary. The previously described search is
repeated with the remaining part of the input realm, if any. If
nothing remains, the realm encoding is complete.
When a matching string is found in the dictionary, the matching part
of the input realm is encoded using the "label index" tag. The
matching part of the input realm is removed, and the search is
repeated with the remaining part of the input realm, if any. If
nothing remains, the octet value of zero (0) is inserted to mark the
end of the encoded realm.
The search algorithm also maintains the "longest non-matching string"
for each input realm. Each time the search in the dictionary fails
and a new label gets encoded using the "label length" tag and
inserted into the dictionary, the "longest non-matching string" is
concatenated by this label, including the separating "." (dot, i.e.,
hexadecimal 0x2e). When a match is found in the dictionary, the
"longest non-matching string" is reset (i.e., emptied). Once the
whole input realm has been processed and encoded, all possible
suffixes longer than one label are taken from the string and inserted
into the dictionary.
4.2.3. Encoding Example
This section shows an example of how to encode a set of realms using
the specified realm compression algorithm. For example, a message
might need to compress the realms "foo.example.com",
"bar.foo.example.com", "buz.foo.example.org", "example.com", and
"bar.example.com.org". The following example shows the processing of
input realms on the left-hand side and the contents of the dictionary
on the right-hand side. The example uses hexadecimal representation
of numbers.
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COMPRESSOR: DICTIONARY:
1) Input "foo.example.com"
Search("foo.example.com")
Search("foo.example")
Search("foo")
Encode(0x03,'f','o','o') 0x00 "foo"
+-> "longest non-matching string" = "foo"
Search("example.com")
Search("example")
Encode(0x07,'e','x','a','m','p','l','e') 0x01 "example"
+-> "longest non-matching string" = "foo.example"
Search("com")
Encode(0x03,'c','o','m') 0x02 "com"
+-> "longest non-matching string" = "foo.example.com"
0x03 "foo.example.com"
0x04 "example.com"
Encode(0x00)
2) Input "bar.foo.example.com"
Search("bar.foo.example.com")
Search("bar.foo.example")
Search("bar.foo")
Search("bar")
Encode(0x03,'b','a','r') 0x05 "bar"
+-> "longest non-matching string" = "bar"
Search("foo.example.com") -> match to 0x03
Encode(0x83)
+-> "longest non-matching string" = NUL
Encode(0x00)
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3) Input "buz.foo.example.org"
Search("buz.foo.example.org")
Search("buz.foo.example")
Search("buz.foo")
Search("buz")
Encode(0x03,'b','u','z') 0x06 "buz"
+-> "longest non-matching string" = "buz"
Search("foo.example.org")
Search("foo.example")
Search("foo") -> match to 0x00
Encode(0x80)
+-> "longest non-matching string" = NUL
Search("example.org")
Search("example") -> match to 0x01
Encode(0x81)
+-> "longest non-matching string" = NUL
Search("org")
Encode(0x03,'o','r','g') 0x07 "org"
+-> "longest non-matching string" = "org"
Encode(0x00)
4) Input "example.com"
Search("example.com") -> match to 0x04
Encode(0x84)
Encode(0x00)
5) Input "bar.example.com.org"
Search("bar.example.com.org")
Search("bar.example.com")
Search("bar.example")
Search("bar") -> match to 0x05
Encode(0x85)
Search("example.com.org")
Search("example.com") -> match to 0x04
Encode(0x84)
Search("org") -> match to 0x07
Encode(0x87)
Encode(0x00)
As can be seen from the example, due to the greedy approach of
encoding matches, the search algorithm and the dictionary update
function are not the most optimal. However, we do not claim that the
algorithm would be the most efficient. It functions efficiently
enough for most inputs. In this example, the original input realm
data was 79 octets, and the compressed output, excluding the end
mark, is 35 octets.
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RFC 6521 HAaRO February 2012
5. New Mobile IPv4 Messages and Extensions
This section describes the construction of all new information
elements.
5.1. Mobile Router Route Optimization Capability Extension
This skippable extension MAY be sent by an MR to an HA in the
Registration Request message.
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type | Length | Subtype |A|R|S|O| Rsvd |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
~ Optional Mobile Router HoA ~
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Type 153 (skippable); if the HA does not support route
optimization advertisements, it can ignore this request and
simply not include any information in the reply. "short"
extension format.
Subtype 1
Reserved Set to zero; MUST be ignored on reception.
A Advertise my networks. If the 'A' bit is set, the HA is
allowed to advertise the networks managed by this MR to
other MRs. This also indicates that the MR is capable of
receiving route optimization Registration Requests. In
effect, this allows the MR to work in the CR role.
R Request mobile network information. If the 'R' bit is set,
the HA MAY respond with information about mobile networks
in the same domain.
S Solicit prefixes managed by a specific MR. The MR is
specified in the Optional Mobile Router HoA field.
O Explicitly specify that the requesting router is only able
to initiate outgoing connections and not accept any
incoming connections, due to a NAT device, stateful
firewall, or similar issue on any interface. This is
reflected by the HA in the reply and distributed in Prefix
Advertisements to other MRs.
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RFC 6521 HAaRO February 2012
Optional Mobile Router HoA
Solicited mobile router's home address. This field is only
included if the 'S' flag is set.
5.2. Route Optimization Reply
This non-skippable extension MUST be sent by an HA to an MR in the
Registration Reply message, if the MR indicated support for route
optimization in the registration message and the HA supports route
optimization.
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type | Length | Subtype |O|N|S| Code |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Type 49 (non-skippable); "short" extension format.
Subtype 1
O The 'O' flag in the Mobile Router Route Optimization
Capability Extension was set during registration.
N NAT was detected by the HA. This informs the MR that it is
located behind a NAT. The detection procedure is specified
in RFC 3519 [RFC3519] and is based on the discrepancy
between the registration packet's source address and
indicated CoA. The MR can use this information to make
decisions about route optimization strategy.
S Responding to a solicitation. If the 'S' bit was present
in the MR's Route Optimization Capability Extension
(Section 5.1), this bit is set; otherwise, it is unset.
The Reply code indicates whether route optimization has been
accepted. Values of 0..15 indicate assent, and values 16..63
indicate that route optimization is not done.
0 Will do route optimization.
16 Route optimization declined; reason unspecified.
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RFC 6521 HAaRO February 2012
5.3. Mobile-Correspondent Authentication Extension
The Mobile-Correspondent Authentication Extension is included in
Registration Requests sent from the MR to the CR. The existence of
this extension indicates that the message is not destined to an HA,
but another MR. The format is similar to the other authentication
extensions defined in [RFC5944], with Security Parameter Indexes
(SPIs) replaced by nonce indexes.
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type | Length | Subtype | Reserved |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Home Nonce Index | Care-of Nonce Index |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Authenticator... ~
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
The Home Nonce Index field tells the CR which nonce value to use when
producing the home keygen token. The Care-of Nonce Index field is
ignored in requests to remove a binding. Otherwise, it tells the CR
which nonce value to use when producing the care-of keygen token. If
using a pre-shared key (KRm), the indexes may be set to zero and are
ignored on reception.
Type 49 (non-skippable); "short" extension format.
Subtype 2
Reserved Set to zero; MUST be ignored on reception.
Home Nonce Index
Home Nonce Index in use. If using a pre-shared KRm, set to
zero and ignored on reception.
Care-of Nonce Index
Care-of Nonce Index in use. If using a pre-shared KRm, set
to zero and ignored on reception.
Authenticator
Authenticator field, by default constructed with
First (128, HMAC_SHA1 (KRm, Protected Data)).
Makela & Korhonen Experimental [Page 32]
RFC 6521 HAaRO February 2012
The protected data, just like in other cases where the Authenticator
field is used, consists of
o the UDP payload (i.e., the Registration Request or Registration
Reply data),
o all prior extensions in their entirety, and
o the Type, Length, Home Nonce Index, and Care-of Nonce Index of
this extension.
5.4. Care-of Address Extension
The Care-of Address Extension is added to a Registration Reply sent
by the CR to inform the MR of the upcoming tunnel endpoint.
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type | Length | Subtype | Reserved |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
1..n times the following information structure
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Care-of Address |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Type 49 (non-skippable); "short" extension format.
Length Total length of the packet. When processing the
information structures, if Length octets have been reached,
this is an indication that the final information structure
was reached as well.
Subtype 3
Care-of Address
Care-of address(es) that may be used for a tunnel with the
MR, in order of priority. Multiple CoAs MAY be listed to
facilitate faster NAT traversal processing.
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RFC 6521 HAaRO February 2012
5.5. Route Optimization Prefix Advertisement Extension
This non-skippable extension MAY be sent by an HA to an MR in the
Registration Reply message. This extension is only included when
explicitly requested by the MR in the Registration Request message,
setting the 'R' flag of the Mobile Router Route Optimization
Capability Extension. Implicit prioritization of prefixes is caused
by the order of extensions.
The extension contains a sequence of information structures. An
information structure may consist of either an MR HoA or a network
prefix. Any network prefixes following an MR HoA are owned by that
MR. An MR HoA MUST be first in the sequence, since one cannot have
prefixes without an MR.
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type | Subtype | Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
1..n times the following information structure
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|D|M| PLen/Info | Optional Mobile Router HoA (4 octets) ~
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
~ | Optional Prefix (1, 2, 3, or 4 octets) ~
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
~ Realm (1..n characters) ~
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Type 50 (non-skippable); "long" extension format.
Subtype 1
Length Total length of the packet. When processing the
information structures, if Length octets have been reached,
this is an indication that the final information structure
was reached as well.
D Delta. If D = 1, the prefix is a delta from the last
Prefix, where D = 0. MUST be zero on the first information
structure containing a Prefix; MAY be zero or one on
subsequent information structures. If D = 1, the Prefix
field is one octet in length. See Section 4.1 for details.
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M Mobile Router HoA bit. If M = 1, the next field is Mobile
Router HoA, and Prefix and Realm are omitted. If M = 0,
the next field is Prefix followed by Realm, and Mobile
Router HoA is omitted. For the first information
structure, M MUST be set to 1. If M = 1, the 'D' bit is
set to zero and ignored upon reception.
PLen/Info
This field is interpreted differently, depending on whether
the 'M' bit is set or not. If M = 0, the field is
considered to be the PLen field, and the contents indicate
the length of the advertised prefix. The 6 bits allow for
values from 0 to 63, of which 33-63 are illegal. If M = 1,
the field is considered to be the Info field. Permissible
values are 0 to indicate no specific information, or 1 to
indicate "outbound connections only". This indicates that
the target MR can only initiate, not receive, connections
on any of its interfaces (apart from the reverse tunnel to
the HA). This is set if the MR has explicitly requested it
via the 'O' flag in the Mobile Router Route Optimization
Capability Extension (Section 5.1).
Mobile Router HoA
The mobile router's home address. All prefixes in the
following information structures where M = 0 are maintained
by this MR. This field is present only when M = 1.
Prefix The IPv4 prefix advertised. If D = 0, the field length is
PLen bits, rounded up to the nearest full octet. Least-
significant bits starting off PLen (and that are zeros) are
omitted. If D = 1, the field length is one octet. This
field is present only when M = 0.
Realm The Realm that is associated with the advertised Mobile
Router HoA and prefix. If empty, MUST be set to '\0'. For
realm encoding and an optional compression scheme, refer to
Section 4.2. This field is present only when M = 0.
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RFC 6521 HAaRO February 2012
5.6. Home Test Init Message
This message is sent from the MR to the CR when performing the RR
procedure. The source and destination IP addresses are set to the
MR's HoA and the CR's HoA, respectively. The UDP source port MAY be
randomly chosen. The UDP destination port is 434.
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type | Reserved | |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +
| Home Init Cookie |
+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Type 24
Reserved Set to zero; MUST be ignored on reception.
Home Init Cookie
64-bit field that contains a random value, the Home Init
Cookie.
5.7. Care-of Test Init Message
This message is sent from the MR to the CR when performing the RR
procedure. The source and destination IP addresses are set to the
MR's CoA and the CR's HoA, respectively. The UDP source port MAY be
randomly chosen. The UDP destination port is 434.
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type | Reserved | |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +
| Care-of Init Cookie |
+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Type 25
Reserved Set to zero; MUST be ignored on reception.
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Care-of Init Cookie
64-bit field that contains a random value, the Care-of Init
Cookie.
5.8. Home Test Message
This message is sent from the CR to the MR when performing the RR
procedure as a reply to the Home Test Init message. The source and
destination IP addresses, as well as UDP ports, are the reverse of
those in the Home Test Init message for which this message is
constructed. As such, the UDP source port is always 434.
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type | Reserved | Nonce Index |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
+ Home Init Cookie +
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
+ Home Keygen Token +
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Type 26
Reserved Set to zero; MUST be ignored on reception.
Nonce Index
This field will be echoed back by the MR to the CR in a
subsequent Registration Request's authentication extension.
Home Init Cookie
64-bit field that contains a random value, the Home Init
Cookie.
Home Keygen Token
This field contains the 64-bit home keygen token used in
the RR procedure. Generated from cookie + nonce.
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RFC 6521 HAaRO February 2012
5.9. Care-of Test Message
This message is sent from the CR to the MR when performing the RR
procedure as a reply to the Care-of Test Init message. The source
and destination IP addresses, as well as UDP ports, are the reverse
of those in the Care-of Test Init message for which this message is
constructed. As such, the UDP source port is always 434.
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type | Reserved | Nonce Index |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
+ Care-of Init Cookie +
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
+ Care-of Keygen Token +
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Type 27
Reserved Set to zero; MUST be ignored on reception.
Care-of Nonce Index
This field will be echoed back by the MR to the CR in a
subsequent Registration Request's authentication extension.
Care-of Init Cookie
64-bit field that contains a random value, the Care-of Init
Cookie.
Care-of Keygen Token
This field contains the 64-bit care-of keygen token used in
the RR procedure. Generated from cookie + nonce.
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6. Special Considerations
6.1. NATs and Stateful Firewalls
Mechanisms described in Mobile IP NAT traversal [RFC3519] allow the
HA to work with MRs situated behind a NAT device or a stateful
firewall. Furthermore, the HA may also detect whether a NAT device
is located between the mobile node and the HA. The MR may also
explicitly state that it is behind a NAT or firewall on all
interfaces, and this information is passed on to the other MRs with
the Info field in the Route Optimization Prefix Advertisement
Extension (Section 5.5). The HA may also detect NAT and inform the
registering MR via the 'N' flag in the Route Optimization Reply
Extension (Section 5.2). In the case where one or both of the
routers is known to be behind a NAT or is similarly impaired (not
able to accept incoming connections), the tunnel establishment
procedure needs to take this into account.
In the case where the MR is behind a NAT (or firewall) and the CR is
not, the MR will, when the tunnel has been established, send
keepalive messages (ICMP echo requests) through the tunnel. Until a
reply has been received, the tunnel SHOULD NOT be considered active.
Once a reply has been received, NAT mapping is in place, and traffic
can be sent.
The source address may change due to NAT in CoTI and Registration
Request messages. This does not affect the process -- the hash
values are calculated by the translated address, and the Registration
Request will also appear from the same translated address.
Unlike communication with the HA, in the case of route optimization,
the path used for signaling is not used for tunneled packets, as
signaling always uses HoAs, and the MR CR tunnel is from CoA to
CoA. It is assumed that even though port numbers may change, NAT
processing rarely allocates more than one external IP address to a
single internal address; thus, the IP address seen in the
Registration Request and tunnel packets remains the same. However,
the UDP source port number may be different in the Registration
Request and incoming tunnel packets, due to port translation. This
must not cause an error situation -- the CR MUST be able to accept
tunneling packets from a different UDP source port than what was used
in the Registration Request.
Since MRs may have multiple interfaces connecting to several
different networks, it might be possible that specific MRs may only
be able to perform route optimization using specific CoA pairs,
obtained from specific networks -- for example, in a case where two
MRs have an interface behind the same NAT. A similar case may be
Makela & Korhonen Experimental [Page 39]
RFC 6521 HAaRO February 2012
applicable to nested NATs. In such cases, the MR MAY attempt to
detect eligible CoA pairs by performing a registration and attempting
to establish a tunnel (sending keepalives) with each CoA listed in
the Registration Reply's Care-of Address Extension. The eligible
pairs should be recorded in the Route Optimization Cache. If a
tunnel cannot be established with any CoAs, the MR MAY attempt to
repeat the procedure with alternative interfaces. The above
information on network topology can also be configured on the MRs
either statically or via some external feedback mechanism.
If both the MR and the CR are behind two separate NATs, some sort of
proxy or hole-punching technique may be applicable. This is out of
scope for this document.
6.2. Handling of Concurrent Handovers
If both the MR and the CR move at the same time, this causes no
issues from the signaling perspective, as all requests are always
sent from a CoA to HoAs. Thus, the recipient will always receive the
request and can send the reply. This applies even in break-before-
make situations where both the MR and the CR get disconnected at the
same time -- once the connectivity is restored, one endpoint of the
signaling messages is always the HoA of the respective router, and it
is up to the HA to provide reachability.
6.3. Foreign Agents
Since foreign agents have been dropped from work related to Network
Mobility for Mobile IPv4, they are not considered here.
6.4. Multiple Home Agents
MRs can negotiate and perform route optimization without the
assistance of an HA -- if they can discover each other's existence
and thus know where to send registration messages. This document
only addresses a logically single HA that distributes network prefix
information to the MRs. Problems arise from possible trust
relationships; in this document, the HA serves as a way to provide
verification that a specific network is managed by a specific router.
If route optimization is desired between nodes attached to separate
HAs, there are several possibilities. Note that standard high-
availability redundancy protocols, such as the Virtual Router
Redundancy Protocol (VRRP), can be utilized; however, in such a case,
the HA is still a single logical entity, even if it consists of more
than a single node.
Makela & Korhonen Experimental [Page 40]
RFC 6521 HAaRO February 2012
Several possibilities exist for achieving route optimization between
MRs attached to separate HAs, such as a new discovery/probing
protocol or routing protocol between HAs or DNS SRV records, or a
common Authentication, Authorization, and Accounting (AAA)
architecture. There is already a framework for HA to retrieve
information from AAA, so it can be considered the most viable
possibility. See Section 6.6 for information on a possible way to
generalize the method.
Any discovery/probing protocols are out of scope for this document.
6.5. Mutualness of Route Optimization
The procedure as specified is asymmetric; that is, if bidirectional
route optimization is desired while maintaining consistency, the
route optimization (RR check and registration) has to be performed in
both directions, but this is not strictly necessary. This is
primarily a policy decision, depending on how often the mobile
prefixes are reconfigured.
Consider the case where two networks, A and B, are handled by MRs A
and B, respectively. If the routers are set up in such a fashion
that route optimization is triggered when the router is forwarding a
packet destined to a network prefix in the Route Optimization Cache,
the following occurs if a node in network A starts sending ICMP echo
requests (ping packets) to a node in network B.
MR A sees the incoming ICMP echo request packet from the local
network destined to network B. Since network B exists in MR A's Route
Optimization Cache, the route optimization process is triggered. The
original packet is forwarded via the reverse tunnel toward the HA as
normal.
MR A completes the RR procedure and registration with MR B, which
thus becomes a CR for MR A. A tunnel is created between the routers.
MR B updates its routing tables so that network A is reachable via
the MR A MR B tunnel.
The traffic pattern is now such that packets from network B to
network A are sent over the direct tunnel, but the packets from A to
B are transmitted via the HA and reverse tunnels. The echo reply
that the node in network B sends toward network A triggers the route
optimization at MR B in similar fashion. As such, MR B now performs
its own registration toward MR A. Upon completion, MR B notices that
a tunnel to MR A already exists, and updates its routing table so
that network A is now reachable via the (existing) MR A MR B
tunnel. From this point onward, traffic is bidirectional.
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In this scenario, if MR A does NOT wait for a separate route
optimization process (RR check and registration) from MR B, but
instead simply updates its routing table to reach network B via the
tunnel, problems may arise if MR B has started to manage another
network, B', before the information has been propagated to MR A. The
end result is that MR B starts to receive packets from network A to
network B' via the HA and to network B via the direct tunnel. If
reverse path checking or a similar mechanism is in use on MR B, some
of the packets from network A could be black-holed.
Whether to perform this mutual registration or not thus depends on
the situation, and whether MRs are going to start managing additional
network prefixes during operation.
6.6. Extensibility
The design considerations include several mechanisms that might not
be strictly necessary if route optimization were only desired between
individual customer sites in a managed network. The registration
procedure (with the optional return routability part), which allows
CRs to learn an MR's CoAs, is not strictly necessary; the CoAs could
have been provided by the HA directly.
However, this approach allows the method to be extended to a more
generic route optimization. The primary driver for having an HA to
work as a centralized information distributer is to provide MRs with
not only the knowledge of the other routers, but with information on
which networks are managed by which routers.
The HA provides the information on all feasible nodes with which it
is possible to establish route optimization. If representing a whole
mobile network is not necessary -- in effect, the typical mobile node
correspondent node situation -- the mechanisms in this document
work just as well; the only problem is discovering whether the target
correspondent node can provide route optimization capability. This
can be performed by not including any prefixes in the information
extension -- just the HoA of the MR.
In addition, with route optimization for a single node, checks for
whether an MR is allowed to represent specific networks are
unnecessary, since there are none.
Correspondent node/router discovery protocols (whether they are based
on probing or a centralized directory beyond the single HA) are
outside the scope of this document.
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6.7. Load Balancing
This design simply provides the possibility of creating optimal paths
between MRs; it doesn't dictate what the user traffic using these
paths should be. One possible approach in helping facilitate load
balancing and utilizing all available paths is presented in
[MIPv4FLOW], which effectively allows for multiple CoAs for a single
HoA. In addition, per-tunnel load balancing is possible by using
separate CoAs for separate tunnels.
7. Scalability
Home agent-assisted route optimization scalability issues stem from
the general Mobile IPv4 architecture, which is based on tunnels.
Creating, maintaining, and destroying tunnel interfaces can cause
load on the MRs. However, the MRs can always fall back to normal,
reverse-tunneled routing if resource constraints are apparent.
If there are a large number of optimization-capable prefixes,
maintaining state for all of these may be an issue also, due to
limits on routing table sizes.
Registration responses from the HA to the MR may provide information
on a large number of network prefixes. If thousands of networks are
involved, the Registration Reply messages are bound to grow very
large. The prefix and realm compression mechanisms defined in
Section 4 mitigate this problem to an extent. There will, however,
be some practical upper limit, after which some other delivery
mechanism for the prefix information will be needed.
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8. Example Signaling Scenarios
8.1. Registration Request
The following example assumes that there are three mobile routers --
MR A, MR B, and MR C -- each managing network prefixes A, B, and C.
At the beginning, no networks are registered with the HA. Any AAA
processing at the HA is omitted from the diagram.
+--------+ +--------+ +--------+ +--------------+
| [MR A] | | [MR B] | | [MR C] | | [Home Agent] |
+--------+ +--------+ +--------+ +--------------+
| | | |
x------------------------------->| Registration Request
| | | | includes Mobile Router
| | | | Route Optimization
| | | | Capability Extension
| | | |
|| Registration Request similar
| | | | to the one sent by MR A
| | | |
| || Registration Request similar
| | | | to the one sent by MR A
| | | |
| | |
RFC 6521 HAaRO February 2012
8.2. Route Optimization with Return Routability
The following example has the same network setup as that in
Section 8.1 -- three MRs, each corresponding to their respective
network. Node A is in network A, and Node C is in network C.
At the beginning, none of the MRs know each other's KRms. If the
KRms were pre-shared or provisioned with some other method, the
Return Routability messages could be omitted. Signaling as described
in Section 8.1 has occurred; thus, MR A is not aware of the other
networks, and MR C is aware of networks A and B.
======= Traffic inside Mobile IP tunnel to/from HA
=-=-=-= Traffic inside Mobile IP tunnel between MRs
------- Traffic outside Mobile IP tunnel
+----------+ +--------+ +------+ +--------+ +----------+
| [Node A] | | [MR A] | | [HA] | | [MR C] | | [Node C] |
+----------+ +--------+ +------+ +--------+ +----------+
| | | | |
x------------O==========O=========O------>| Mobile Router A is
| | | | | unaware of network C;
| | | | | thus, nothing happens
| | | | |
|| | CoT
| x==========O========>| | HoT
| | | | |
| | | | | KRm between MR A C
| | | | | established
| | | | |
| |<=========O---------x | Registration Request
| | | | |
| x--------->| | | Registration Request
| | | | | to HA due to MR A
| | | | | being unaware of
| | | | | network C.
| | | | | Solicit bit set.
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RFC 6521 HAaRO February 2012
| | | | |
| || | Registration Reply
| | | | | includes MR A's CoA in
| | | | | Care-of Address
| | | | | Extension
| | | | |
| |<= = = = =O= = = ==>| | Optional mutual
| | | | | registration from
| | | | | MR A to MR C
| | | | | (same procedure as above,
| | | | | multiple packets);
| | | | | possible keepalive checks
| | | | |
| A
| | | | | routed to direct tunnel
| | | | | at MR C, based on
| | | | | MR C now knowing MR A's
| | | | | CoA and tunnel being up
| | | | |
x------------O=-=-=-==-=-=-=-==-=-O------>| Packet from Node A -> C
| | | | | routed to direct tunnel
| | | | | at MR A, based on MR A
| | | | | now knowing MR C's CoA
| | | | | and tunnel being up
8.3. Handovers
In this signaling example, MR C changes its CoA while route
optimization between MR A and MR C is operating and data is being
transferred. Cases where the handover is graceful ("make before
break") and ungraceful ("break before make") both occur in similar
fashion, except that in the graceful version no packets are lost.
This diagram considers the case where MR C gets immediate
notification of lost connectivity, e.g., due to a link status
indication. MR A would eventually notice the breakdown, due to
keepalive messages failing.
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======= Traffic inside Mobile IP tunnel to/from HA
=-=-=-= Traffic inside Mobile IP tunnel between MRs
------- Traffic outside Mobile IP tunnel
+----------+ +--------+ +------+ +--------+ +----------+
| [Node A] | | [MR A] | | [HA] | | [MR C] | | [Node C] |
+----------+ +--------+ +------+ +--------+ +----------+
| | | | |
x------------O=-=-=-==-=-=-=-==-=-O------>| Nodes A and C are
|x | | Traffic from A -> C
| | | | | lost, and
| | | x<=-=-O-------x vice versa
| | | | |
| | || | Registration Reply
| | | | |
x------------O=-=-=-==-=-=-=->x | | Traffic from A -> C lost
| | | | | (reverts to routing via
| | | | | HA if enough keepalives
| | | | | fail)
| | | | |
| A
| | | | | sent via HA
| | | | |
| O<=========O---------x | CoTI message
| | | | | (partial RR check)
| | | | |
| x==========O-------->| | CoT message
| | | | |
| |<=========O---------x | Registration Request
| | | | | reusing newly calculated
| | | | | KRm
| | | | |
| x==========O-------->| | Registration Reply
| | | | |
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| O<=-=-=-=-=-=-=-=-=-=x | First keepalive check if
| | | | | using UDP encapsulation;
| | | | | also creates holes in
| x=-=-=-=-=-=-=-=-=-=>| | firewalls
| | | | |
| | | | |
x------------O=-=-=-==-=-=-=-==-=-O------>| Traffic from A -> C
| | | | | forwarded directly again
| | | | |
| A
| | | | | switches back to direct
| | | | | tunnel
| | | | |
9. Protocol Constants
MAX_NONCE_LIFETIME 240 seconds
MAX_TOKEN_LIFETIME 210 seconds
MAX_UPDATE_RATE 5 times
10. IANA Considerations
IANA has assigned rules for the existing registries "Mobile IP
Message Types" and "Extensions to Mobile IP Registration Messages",
specified in RFC 5944 [RFC5944]. New Mobile IP message types and
extension code allocations have been made for the messages and
extensions listed in Section 5.
The route optimization authentication processing requires four new
message type numbers. The new Mobile IP Message types are listed
below, in Table 1.
+-------+---------------------------+
| Value | Name |
+-------+---------------------------+
| 24 | Home Test Init message |
| 25 | Care-of Test Init message |
| 26 | Home Test message |
| 27 | Care-of Test message |
+-------+---------------------------+
Table 1: New Values and Names for Mobile IP Message Types
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Three new registration message extension types are required and
listed in Table 2. The first type, 153, is skippable and has been
allocated from range 128-255. The other two, 49 and 50, are
non-skippable and have been allocated from range 0-127, with 49 being
of the "short" format and 50 being of the "long" format. None of the
messages are permitted for notification messages.
+--------------+---------------------------------------------+
| Value | Name |
+--------------+---------------------------------------------+
| 153, 128-255 | Mobile Router Route Optimization Indication |
| 49, 0-127 | Route Optimization Extensions |
| 50, 0-127 | Route Optimization Data |
+--------------+---------------------------------------------+
Table 2: New Values and Names for Extensions in Mobile IP
Registration Messages
In addition, the registry "Code Values for Mobile IP Registration
Reply Messages" has been modified. A new success code, 2, should be
allocated as follows:
2 Concurrent registration (pre-accept)
In addition, a new allocation range has been created as "Error Codes
from the Correspondent Node", subject to the policy of Expert Review
[RFC5226]. The range is 201-210. Three new Registration Reply codes
have been allocated from this range. They are specified in Table 3,
below:
+-------+-----------------------------+
| Value | Name |
+-------+-----------------------------+
| 201 | Expired Home nonce Index |
| 202 | Expired Care-of nonce Index |
| 203 | Expired nonces |
+-------+-----------------------------+
Table 3: New Code Values and Names for Mobile IP
Registration Reply Messages
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Three new number spaces were required for the subtypes of the
extensions in Table 2. A new registry, named "Route Optimization
Types and Subtypes", has been created with an allocation policy of
RFC Required [RFC5226]. The registration entries include Type,
Subtype, and Name. Type and Subtype have a range of 0-255. Types
are references to registration message extension types. Subtypes are
allocated initially as in Table 4, below:
+------+---------+--------------------------------------------------+
| Type | Subtype | Name |
+------+---------+--------------------------------------------------+
| 153 | 0 | Reserved |
| 153 | 1 | Mobile Router Route Optimization Capability |
| | | Extension |
| 49 | 0 | Reserved |
| 49 | 1 | Route Optimization Reply |
| 49 | 2 | Mobile-Correspondent Authentication Extension |
| 49 | 3 | Care-of Address Extension |
| 50 | 0 | Reserved |
| 50 | 1 | Route Optimization Prefix Advertisement |
| | | Extension |
+------+---------+--------------------------------------------------+
Table 4: Initial Values and Names for Registry Route Optimization
Types and Subtypes
11. Security Considerations
There are two primary security issues: One issue relates to the RR
check, which establishes that a specific CoA is, indeed, managed by a
specific HoA. The other issue is trust relationships and an
arbitrary router claiming to represent an arbitrary network.
The end-user traffic can be protected using normal IPsec mechanisms.
11.1. Return Routability
The RR check's security has been vetted with Mobile IPv6. There are
no major differences, apart from two issues: connectivity check and
replay attack protection. The connectivity check is conducted with a
separate ICMP message exchange. Replay attack protection is achieved
with Mobile IPv4 timestamps in the Registration Request's
Identification field, in contrast to the sequence numbers used in
Mobile IPv6.
The RR procedure does not establish any kind of state information on
the CR; this mitigates denial-of-service attacks. State information
is only maintained after a Registration Request has been accepted.
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11.2. Trust Relationships
The network of trust relationships in home agent-assisted route
optimization solves possible trust issues: An arbitrary CR can trust
an arbitrary MR that it is indeed the proper route to reach an
arbitrary mobile network.
It is assumed that all MRs have a trust relationship with the HA.
Thus, they trust information provided by the HA.
The HA provides information matching HoAs and network prefixes. Each
MR trusts this information.
MRs may perform the RR procedure between each other. This creates a
trusted association between the MR's HoA and CoA. The MR also claims
to represent a specific network. This information is not trustworthy
as such.
The claim can be verified by checking the HoA network prefix
information received, either earlier, or due to an on-demand request,
from the HA. If they match, the MR's claim is authentic. If the
network is considered trusted, a policy decision can be made to skip
this check. Exact definitions on situations where such decisions can
be made are out of scope for this document. The RECOMMENDED general
practice is to perform the check.
12. Acknowledgements
Thanks to Alexandru Petrescu for constructive comments and support.
Thanks to Jyrki Soini and Kari Laihonen for initial reviews. This
work was supported by TEKES as part of the Future Internet program of
TIVIT (Finnish Strategic Centre for Science, Technology and
Innovation in the field of ICT).
13. References
13.1. Normative References
[RFC2003] Perkins, C., "IP Encapsulation within IP", RFC 2003,
October 1996.
[RFC2004] Perkins, C., "Minimal Encapsulation within IP",
RFC 2004, October 1996.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
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RFC 6521 HAaRO February 2012
[RFC2784] Farinacci, D., Li, T., Hanks, S., Meyer, D., and P.
Traina, "Generic Routing Encapsulation (GRE)", RFC 2784,
March 2000.
[RFC3519] Levkowetz, H. and S. Vaarala, "Mobile IP Traversal of
Network Address Translation (NAT) Devices", RFC 3519,
April 2003.
[RFC5177] Leung, K., Dommety, G., Narayanan, V., and A. Petrescu,
"Network Mobility (NEMO) Extensions for Mobile IPv4",
RFC 5177, April 2008.
[RFC5226] Narten, T. and H. Alvestrand, "Guidelines for Writing an
IANA Considerations Section in RFCs", BCP 26, RFC 5226,
May 2008.
[RFC5944] Perkins, C., Ed., "IP Mobility Support for IPv4,
Revised", RFC 5944, November 2010.
13.2. Informative References
[MIP-RO] Perkins, C. and D. Johnson, "Route Optimization in
Mobile IP", Work in Progress, September 2001.
[MIPv4FLOW] Gundavelli, S., Ed., Leung, K., Tsirtsis, G., Soliman,
H., and A. Petrescu, "Flow Binding Support for Mobile
IPv4", Work in Progress, February 2012.
[RFC1035] Mockapetris, P., "Domain names - implementation and
specification", STD 13, RFC 1035, November 1987.
[RFC3543] Glass, S. and M. Chandra, "Registration Revocation in
Mobile IPv4", RFC 3543, August 2003.
[RFC4086] Eastlake 3rd, D., Schiller, J., and S. Crocker,
"Randomness Requirements for Security", BCP 106,
RFC 4086, June 2005.
[RFC4282] Aboba, B., Beadles, M., Arkko, J., and P. Eronen, "The
Network Access Identifier", RFC 4282, December 2005.
[RFC6275] Perkins, C., Ed., Johnson, D., and J. Arkko, "Mobility
Support in IPv6", RFC 6275, July 2011.
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Authors' Addresses
Antti Makela
Aalto University
Department of Communications and Networking (Comnet)
P.O. Box 13000
FIN-00076 Aalto
FINLAND
EMail: antti.t.makela@iki.fi
Jouni Korhonen
Nokia Siemens Networks
Linnoitustie 6
FI-02600 Espoo
FINLAND
EMail: jouni.nospam@gmail.com
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